This invention relates to apparatus for indicating, e.g. for an alarm and/or safeguard control purpose, when the value of a sensed parameter of a process or in a system is beyond a predetermined limit.
In many forms of apparatus or process, it is required to hold a parameter above or below a predetermined limit or within certain normal limits. For example in a process where temperature is controlled, the control system may operate to hold the temperature between two levels. If the temperature should rise above the upper level and/or fall below the lower level of the range of levels permissible under the normal control conditions, then it may be required to operate an alarm or to effect a safeguard control operation. Similarly, in a boiler where the water level is controlled between normal limits, if it should fall below the lower level and/or rise above the upper level of these normal limits, then an alarm operation and/or a safeguard control operation may be required. The safeguard control operation or the action which has to be taken on such an alarm may well be a matter of great expense or serious inconvenience. For instance, with a large steam-raising boiler feeding a turbine of an electrical generating plant, it may be necessary to remove all the load from the generator and from the boiler in the event of there being insufficient water in the boiler. With a steam driven turbo-generator, similar requirements may exist if the water level in a condenser should rise because of a failure of an extraction pump. In a process plant in which material is temperature-controlled, an emergency shut down may be very costly in damage to the plant, for example through molten material solidifying. For these reasons, a very high degree of reliability is desirable for alarm or safeguard control apparatus in large plant or process systems such as have been described above. Whilst it is essential that the parameter condition is detected and the alarm or control operation effected, it is important that the alarm or the safeguard control should not normally be effected through any malfunction or failure in the alarm and/or control apparatus.
It is an object of the present invention to provide an improved form of apparatus for alarm or safeguard control operative when the level of a parameter is outside a predetermined limit which will have a high degree of reliability yet wherein safeguards are provided against false alarm or control operation being effected due to faults in the apparatus.